Altered mechanical loading is a contributing factor to low back pain, a condition affecting 80% of the population at some point in life. A plethora of in-vitro studies exist focusing on 6 degree of freedom (dof) testing of functional spinal units (FSU) to obtain a specimen stiffness matrix. Due to differences in the performance of test apparatus and in the technique used to manipulate raw data it is difficult to compare results from different groups. Our primary objective was to develop a standardised technique to benchmark the performance of testing apparatus; a secondary objective was to standardise the data manipulation technique.Abstract
Introduction
Objectives
The hip joint is subjected to cyclic loading during activities of daily living and this can induce micromotion at the bone-implant interfaces of uncemented implants. Osseointegration, which is essential for long term implant survival, will occur when micromotion at these interfaces is below 40µm and may occur up to 150µm [1]. Studies investigating the micromotion of press-fit acetabular cups only report micromotions in one direction. Standard methods also maintain a static cup position throughout testing; usually at the angle of maximum resultant force during gait. Current methods therefore do not take into account the effect of motion of the hip on micromotion of the cup, nor do they investigate all six degree of freedom (DoF) of motion. The aim of this study was to assess press-fit cup micromotion in six DoF under physiological loading when the cup is held statically and moved in flexion-extension. A cementless acetabular cup (Trident, Stryker) was implanted into polyurethane foam blocks (Sawbones, density = 0.48g/cm³) with a 1mm press-fit. The blocks were manufactured to replicate important anatomical features, which model the acetabulum (Figure 1). A six DoF measurement system was rigidly attached to the bottom of the cup through the dome screw hole and micromotion was measured using six LVDT sensors (Figure 2). The micromotion of the cup was measured under three conditions. Firstly, the cup was tested statically at 30° flexion, representing heel strike during gait; secondly, under dynamic motion simulating gait (30° flexion to −15° extension; 0.5Hz); and finally, under dynamic motion simulating stair climb (45° flexion to −15° extension; 0.5Hz) [2]. For all conditions, the cup was cyclically loaded to a peak load of 2.0kN for 1000 cycles at 1Hz. The loading cycles were synchronised with the flexion-extension movement in order to achieve a loading peak at both heel strike and toe-off positions.Introduction
Methods
